As Adam indicates, we haven't seen many like this, so, while you and I both want to make sure we're ready for this in the future and understand exactly what it's trying to ask, it's practically very unlikely we'll see anything like this on a future test. With that in mind, let's look at this one.
The question is indicating that the explanation is "only a partial one". If the author thought his explanation WAS only partial, then how would the stimulus have been different? I think it would have said something like "At least one reason that a hard surface makes for faster running" or "running is faster on hard surfaces in part because" or something similar. That the author did not include such qualifying language is a pretty good indication that the author thinks his explanation is complete.
I think another unusual thing about this question stem and stimulus combination is that you're not really weakening the conclusion of the argument. Instead, you're weakening the sufficiency of the evidence given to support that conclusion, and not trying to undermine the conclusion itself. In effect, you're weakening an argument that says what the stimulus said, and then adds "And there is no other reason that running is faster on hard surfaces."
We also actually have to strengthen the "hard surfaces make for faster running" conclusion in order to show the explanation is only partial. If we weaken that conclusion, we're not showing that the explanation is only partial - we'd be undermining the explanation entirely, making it NOT EVEN partial.
A question that I think is similar in spirit to this one is here: viewtopic.php?f=660&t=10630
For that question, we're trying to strengthen the argument, but do it in a specific way; in the question we're talking about in this thread, we're trying to weaken the argument, but not in a way that weakens the conclusion. It's odd to have such a constraint on how you weaken or strengthen an argument. Normally, to weaken, you just make the conclusion less likely; to strengthen, you make the conclusion more likely.